Forward osmosis membranes

ABSTRACT

Forward osmosis membranes include an active layer and a thin support layer. A bilayer substrate including a removable backing layer may allow forward osmosis membranes with reduced supporting layer thickness to be processed on existing manufacturing lines.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority under Section 119 to U.S. ProvisionalApplication No. 61/236,441 filed on Aug. 24, 2009, U.S. ProvisionalApplication No. 61/253,786 filed on Oct. 21, 2009, and U.S. ProvisionalApplication No. 61/291,430 filed on Dec. 31, 2009, the entire disclosureof each of which is hereby incorporated herein by reference in itsentirety for all purposes.

FIELD OF THE TECHNOLOGY

One or more aspects relate generally to osmotic separation. Moreparticularly, one or more aspects involve membranes that are useful inengineered osmotic separation processes.

BACKGROUND

Semipermeable membranes are substantially permeable to a liquid andsubstantially impermeable to solutes based on the nature of theirselective barrier. Osmotically driven membrane separation generallyrelies upon driving forces associated with the passage of draw solutesthrough one or more support layers of a membrane used in the separationprocess.

Polymeric membranes used in liquid separations are typically thin-filmcomposite (TFC) membranes which generally include a selective barrier ona porous support structure. Recent development of highly selectivemembranes has been focused primarily on the reverse osmosis (RO)process. Reverse osmosis is a pressure driven process in which theprimary resistance to water flux through the membrane is hydrodynamiconce the osmotic pressure of the solution is overcome by an excess ofhydraulic pressure. Forward osmosis (FO), by contrast, is a diffusiondriven process. The factors affecting water flux in RO and FO processesare different, in turn requiring different membrane structures foroptimum performance.

SUMMARY

Aspects relate generally to forward osmosis membranes and methods ofmaking forward osmosis membranes.

In accordance with one or more embodiments, a method of making a forwardosmosis membrane may comprise providing a support structure including atleast a first layer and a second layer, applying a material to a firstlayer of the support structure to form a membrane support layer,applying a barrier material to the membrane support layer to form theforward osmosis membrane, and releasing the forward osmosis membrane byseparating the first layer of the support structure from the secondlayer of the support structure.

In some embodiments, the support structure may comprise a bilayerstructure. The first layer of the support structure may have a Frazierair permeability of greater than about 50 cfm/ft² min. The materialapplied to the first layer of the support structure may be applied in acoating of between about 5 and 20 g/m². The forward osmosis membrane mayhave an overall thickness of less than about 125 microns. The barriermaterial may comprise a semipermeable material. In at least oneembodiment, the barrier material may comprise a polymer. In somenonlimiting embodiments, the barrier material may comprise a polyamideurea, polypiperazine, or a block co-polymer. The support structure maycomprise a polymeric paper. The support structure may comprise PET orpolypropylene.

In some embodiments, the method may further comprise rewetting theforward osmosis membrane. The method may also further comprise adding anadditive to one or more layers of the membrane. In at least someembodiments, the step of releasing the forward osmosis membrane byseparating the first and second layers of the support structure maycomprise modifying a pore structure in at least a section of the forwardosmosis membrane. The method may further comprise reusing the secondlayer of the support structure.

In accordance with one or more embodiments, a method of making a forwardosmosis membrane may comprise casting a paperless support structure on afabrication belt or drum, depositing a barrier material on the paperlesssupport structure, and delaminating the paperless support structure fromthe fabrication belt to form the forward osmosis membrane.

In some embodiments, the belt or drum may be constructed and arranged toprovide a surface which retains a portion of the support structure.Retained support structure may be removed prior to full rotation of thebelt or drum.

In accordance with one or more embodiments, a method of facilitating aforward osmosis separation operation may comprise providing a supportstructure, applying a material to the support structure to form amembrane support layer, applying a barrier layer to the membrane supportlayer to form a forward osmosis membrane, and configuring the forwardosmosis membrane in a forward osmosis membrane module such that thesupport structure may provide a spacer or a flow channel for a drawsolution or a feed solution supplied to the module during the forwardosmosis separation operation.

In some embodiments, providing the support structure may compriseproviding a bilayer structure. The method may further comprise providinga source of the draw solution.

In accordance with one or more non-limiting embodiments, a forwardosmosis membrane may comprise a fabric layer of less than about 75microns, a support layer of less than about 50 microns applied on thefabric layer, and a barrier layer applied on the support layer. Theforward osmosis membrane may have an overall thickness of less thanabout 125 microns.

In some embodiments, the barrier layer may comprise polyamide. Thesupport layer may comprise PET. The support layer may contain less thanabout 30 g/m² of material overall. The supporting material may beapplied in a coating of between about 8 and 17.5 g/m². The combinedweight of the support layer, support material, and barrier layer overallmay be between about 20 and 40 g/m². The support layer may be made in awet laid process.

In some embodiments, a method may comprise rewetting a forward osmosismembrane by immersion in water containing a solute which enhanceswetting. The solute may be a surfactant. In other embodiments, themethod may further comprise rewetting the membrane by immersion in watercontaining a low surface tension solvent. The solvent may comprise analcohol. In some embodiments, the solvent may comprise isopropylalcohol, ethanol or methanol. In at least one embodiment, the supportstructure is embedded within the hydrophilic material. In someembodiments, the barrier material comprises polyamide.

Still other aspects, embodiments, and advantages of these exemplaryaspects and embodiments, are discussed in detail below. Moreover, it isto be understood that both the foregoing information and the followingdetailed description are merely illustrative examples of various aspectsand embodiments, and are intended to provide an overview or frameworkfor understanding the nature and character of the claimed aspects andembodiments. The accompanying drawings are included to provideillustration and a further understanding of the various aspects andembodiments, and are incorporated in and constitute a part of thisspecification. The drawings, together with the remainder of thespecification, serve to explain principles and operations of thedescribed and claimed aspects and embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of at least one embodiment are discussed below withreference to the accompanying figures. The figures are provided for thepurposes of illustration and explanation and are not intended as adefinition of the limits of the invention. In the figures:

FIGS. 1-6 present data referenced in the accompanying Example.

DETAILED DESCRIPTION

Osmotic separation processes generally involve generating water fluxacross a semi-permeable membrane based on osmotic pressuredifferentials. Solute may be rejected by the membrane and retained oneither side due to the greater permeability of water than the solutewith respect to the selective barrier of the membrane. Solutes may beundesirable and therefore removed from a process stream via membraneseparation for purification, or desirable in which case they may beconcentrated and collected via a membrane separation process.

Membranes may be used in various osmotically driven separation processessuch as but not limited to desalination, wastewater purification andreuse, FO or PRO bioreactors, concentration or dewatering of variousliquid streams, concentration in pharmaceutical and food-gradeapplications, PRO energy generation and energy generation via an osmoticheat engine.

Polymeric membranes typically include a porous support structure thatprovides mechanical and structural support for a selective layer.Membranes may be formed in various shapes including spiral wound, hollowfiber, tubular and flat sheet depending on an intended application.Membrane characteristics should be customized to achieve idealperformance and may vary between specific applications. For example, inFO and PRO applications, the effectiveness of a separation process maybe enhanced by reducing the thickness and tortuosity of the membrane,while increasing its porosity and hydrophilicity, without sacrificingstrength, salt rejection and water permeability properties.

The RO membrane industry has, to date, standardized on a polyethyleneterephthalate (PET) support layer with a polysulfone active coating. ThePET support layer is generally about four mils thick with a basis weightof approximately 80 g/m² and a Frazier air permeability of approximatelyfour cfm/ft²/min Although robust, the PET material generally representsthe most expensive raw material of the membrane and provides little tono benefit to the performance of the RO membrane. When PET is used as asupport structure in other osmotic separation membranes, such as for FOand pressure retarded osmosis (PRO) processes, membrane performance issignificantly impeded.

Thus, reducing the thickness of the support structure may be desirable.Attempting to reduce the thickness or weight of the support material maybe associated with membrane processing problems, however, such as theinability to run the support material through a membrane fabricationline without creasing or wrinkling. In severe cases, membrane webbreakage can occur which may result in significant costs to amanufacturer.

In accordance with one or more embodiments, the manufacture of membraneswith reduced thickness for various engineered osmotic separationprocesses may be facilitated. In at least some embodiments, thethickness of the membrane support structure may be reduced. Thinnersupport structures may be associated with reduced cost, enhanced masstransfer and higher flux within the membrane by reducing resistance tofluid flow and solute diffusion through the membrane support, and anincrease in the amount of active membrane area which may be provided ina separation module. Furthermore, as water standards continue to becomemore stringent, reducing the thickness of the support structure mayreduce the level of various residual chemicals of concern in themembranes, such as dimethylformamide and meta phenylene diamine.

In accordance with one or more embodiments, a bilayer substrate may beprovided to facilitate membrane fabrication. A bilayer substrate mayinclude a membrane support layer which will serve as the membranesupport layer of a final membrane product. The membrane support layer ofthe bilayer substrate may be of reduced thickness compared toconventional membrane support layers while at the same time providing anoverall thickness requisite for membrane manufacturing, including theapplication and processing of a selective layer upon the support layer.In some embodiments, the bilayer support may include a removable backinglayer in addition to the membrane support layer to provide the extrathickness. The removable backing layer may be intended to be separatedfrom the support layer subsequent to membrane fabrication. In otherembodiments, the bilayer substrate may include a backing layer intendedto remain intact subsequent to membrane fabrication. In at least someembodiments, the backing layer may remain connected to the support layerand incorporated into a membrane module.

In accordance with one or more embodiments, a bilayer substrate mayinclude a membrane support layer generally affixed to a removablebacking layer. The membrane support layer may be the support layer of aresultant membrane while the removable backing layer may be largelysacrificial, temporarily providing increased thickness to the supportlayer to facilitate membrane processing. The membrane support layer ofthe bilayer substrate may generally be a light basis weight layer ofreduced thickness in comparison to a conventional membrane supportlayer. In at least one embodiment, the support layer may be PET. In someembodiments, the support layer and backing layer may be made of the samematerial. In other embodiments, they may be made of different materials.The bilayer substrate may be characterized by properties which allow thetwo layers together to perform similar to an existing standard PETsupport layer with respect to strength, resistance to creasing, andgeneral processing in the membrane manufacturing process.

In some nonlimiting embodiments, the backing layer may typically beabout two to about four mils in thickness with a Frazier airpermeability of less than about 6 cfm/ft² min. The top layer, which mayultimately be the membrane support structure as described herein, maytypically be less than about 2 mils in thickness with a Frazier airpermeability of greater than about 100 cfm/ft² min. The forward osmosismembrane may have an overall thickness of less than about 125 microns.The support layer may contain less than about 30 g/m² of materialoverall. The supporting material may be applied in a coating of betweenabout 8 and 17.5 g/m². The combined weight of the support layer, supportmaterial, and barrier layer overall may be between about 20 and 40 g/m².The top support layer may be made in a wet laid process, dry laidprocess, or a woven material. Alternately, the support layer may be madeby deposition in the presence of an electrical field, such as in anelectrospinning method. Materials may include PET or other polymerstypically used in the fabrication of pressure driven membrane supports,and may additionally be designed to have a hydrophilic nature. In someembodiments, the support structure may be a paper, such as a polymericpaper. In some nonlimiting embodiments, the support material may be madeof PET, polypropylene, polysulfone, polyacrylonitrile, or other porouspolymers suitable for creating a support for interfacial polymerizationof a polyamide, polyamide urea, or similar type barrier layer.Hydrophilic additives may be introduced to the support material.

A selective or otherwise active layer may be applied to the supportmaterial of the bilayer substrate during a membrane manufacturingprocess. In some embodiments, a semipermeable layer may be applied asthe active layer. The semipermeable layer may comprise a polymer. Incertain embodiments, the semipermeable layer may comprise a polyamide,such as polyamide urea, a block co-polymer or polypiperazine. In somenonlimiting embodiments, a polysulfone layer may be applied to a PETsupport layer of a bilayer substrate. Multi-layered substrates inaccordance with one or more embodiments may be easier to coat thansingle layer since the substrate is sturdier and thicker and therebyless subject to wrinkling and tearing. Subsequent to membraneprocessing, the backing layer may then be separated and removed. Byusing the bilayer substrate, a membrane with a support layer of reducedthickness may be produced using standard manufacturing equipment andtechniques. In some embodiments, the separation step may be performedprior to application of an active layer.

In accordance with one or more embodiments, disclosed methods may becharacterized by minimal penetration of active material, such aspolysulfone, into the backing layer which may facilitate separation andremoval of the backing layer. Without wishing to be limited to anyparticular theory, the use of multiple layers may mitigate the impact ofstrike-through by blocking excess coating material from penetrating thebacking layer. As such, the backing layer may remain largely intactsubsequent to separation, enabling reuse or recycling of the backinglayer which may offer additional efficiencies and cost savings. In otherembodiments, the backing layer may be largely sacrificial. In otherembodiments, the backing layer may be recycled within the fabricationequipment itself, such as with a rotating belt or drum. In someembodiments, the backing layer or belt may allow sufficient penetrationof the support material so that upon removal of the backing material,beneficial disruption of the pore structure of the support materialoccurs. This may cause the base of the porous support material to have amore open and porous structure than it would have had without thisdisruption. Optimal backing layer characteristics may allow for slightpenetration of the porous support material, without allowing completepenetration, or “strike through”, such that when the backing layer isremoved, the pore structure is opened up without causing defects in thebarrier layer above it.

One or more embodiments may find applicability in the manufacture ofmembranes for FO and PRO processes, as well as offer benefits for themanufacture of membranes used in pressure driven separations such as RO,microfiltration (MF), ultrafiltration (UF) and nanofiltration (NF).

In at least some embodiments, a bilayer substrate may be formed bycasting a support polymer on a substantially high release material suchthat after casting an active barrier layer on the support polymer, thesupport polymer may be peeled from the base material such that onlysupport polymer and the active membrane coating remain. In otherembodiments, a bilayer material may be commercially purchased for use ina membrane manufacturing process.

In accordance with one or more embodiments, the removal of a sacrificialor recyclable backing layer may create a more open pore structure at abase region of the polymer support layer which previously interfacedwith the backing layer prior to removal thereof. This may result inenhanced flux through the membrane. The tearing or other disruption of arelatively closed pore structure at the bottom-most section of theporous polymer support may produce a structure characterized bysignificantly enhancing the porosity and reducing the tortuosity of thesupport structure. In many cases, the pore structure of such a polymericsupport will be substantially open through most of its thickness, butbecome closed at the point where the polymer phase interacted with thebase material. If some of the porous support penetrates into the baselayer, removing this layer may expose a much more open structure aswould be found throughout the porous layer, removing the tight layer atthe base.

In accordance with one or more further embodiments, rather than using abilayer substrate which includes a sacrificial or otherwise removablebacking layer, a bilayer substrate may be implemented which is intendedto wholly become part of a resultant membrane. In these embodiments, asupport polymer may be casted directly onto one or both sides of atricot-type mesh support material. A polyurethane or other adhesive maybe used to bind the support layer to the mesh layer. The support polymermay be PET in certain non-limiting embodiments. The mesh support, suchas a supporting tricot, may conduct fluid flow within a finishedmembrane module. A membrane barrier layer may then be applied on one orboth of these support polymer coatings, forming a final single or doublelayer membrane with a water conducting mesh as its base or core.

In some nonlimiting embodiments, a tricot layer may be laminated to athin PET layer to create a bilayer which may then be processed onexisting membrane manufacturing machines. By combining these layersprior to membrane manufacture the strength of the tricot can be used toprocess the thin PET and the resulting bilayer material does not requireseparation in that the membrane including tricot may be incorporateddirectly into a membrane module to enhance module performance. Withoutwishing to be bound to any particular theory, the tricot backing mayincrease membrane production efficiency since it is relativelyimpervious to web breakage and has superior strength compared to thestandard 4 millimeter PET. This may result in reduced creasing andwrinkling in the manufacturing process, increasing membrane performanceand yields. Further production efficiencies may include reduction infabrication steps by eliminating the need for producing leaf sets oftricot, flow spacers or permeate tubes which are traditionallyseparately incorporated into a completed membrane module.

In accordance with one or more embodiments, a bilayer substrate may beprewetted to improve mass transfer characteristics of the supportpolymer and polymer/fabric interface. A solvent such as with a solventsuch as NMP, DMF, DMSO, triethyl phosphate, dimethyl acetamide, or acombination thereof may be used to prewet. Prewetting may create a moreopen pore structure, cause less occlusion of pores in the polymersupport, enhance polymer porosity by encouraging macrovoid formation,improve pore structure and decrease tortuosity. These properties may berealized and even enhanced by separation of removable backing layer ifused. These properties may be particularly desirable when using bilayerassemblies which are not intended to be separated, such as withsupporting tricot mesh and PET fabric, for example, by preventingexcessive penetration of the polymer into the supporting material.

In accordance with one or more embodiments, a process to manufacture amembrane for osmotically driven membrane processes may include the useof a drive system to transport a bilayer sheet of support material, forexample two layers of PET paper, through a casting machine which maydeposit a polymer in a solvent solution. Tensions may generally bemaintained so as to reduce the possibility of creasing and wrinkling.The bilayer support material may be composed of two layers pressedtogether such that the bottom layer may be either subsequently removedor ultimately used as a membrane fluid channel spacer mesh.

The bilayer support material may be conveyed to a polymer applicationdevice which applies a solution of polymer, for example polysulfone, ina solvent, for example dimethylformamide. Upon coating, the bilayermaterial may enter a quenching bath in which the polymer precipitatesinto the top layer of the bilayer material. The temperature of thequenching bath may vary and may impact one or more properties of aresultant membrane. In at least some preferred nonlimiting embodiments,improved properties of forward osmosis membranes may be associated withquenching bath temperature in the range of 100° F. to 110° F. Thebilayer top layer is designed to allow sufficient penetration of thesolution to result in delamination pressures at which the precipitatedpolymer layer would disengage from the bilayer support material inexcess of about ten psig. The backing layer of the bilayer material incontrast is designed to prevent polymer penetration to allow for the twosupport material layers to be separated after membrane manufacturing.The primary purpose of the backing layer is to prevent creasing andwrinkling of the top layer while processing by providing necessarystrength to allow existing membrane machines to convey the very thinmembrane required for forward osmosis membranes. The remainder of themembrane production is completed using standard rinsing and membranecasting equipment.

Any removable bilayer material implemented may be separated prior to oras part of module fabrication. The separation of the two layers canprovide the additional benefit of opening the pore structure at theinterface of the PET layers further enhancing separation properties.Alternately, the bottom layer of the bilayer material may be integratedinto the membrane module construction serving the role of the spacermesh, for example tricot normally integrated between membrane layers asa fluid conveyance medium.

In accordance with one or more embodiments, the selective barrier in thedisclosed thin-film composite membranes may be a semipermeablethree-dimensional polymer network, such as an aliphatic or aromaticpolyamide, aromatic polyhydrazide, poly-bensimidazolone,polyepiamine/amide, polyepiamine/urea, polyethyleneimine/urea,sulfonated polyfurane, polybenzimidazole, polypiperazine isophtalamide,a polyether, a polyether-urea, apolyester, or a polyimide or a copolymerthereof or a mixture of any of them. In certain embodiments, theselective barrier may be an aromatic or non-aromatic polyamide, such asresidues of a phthaloyl (e.g., isophthaloyl or terephthaloyl) halide, atrimesyl halide, or a mixture thereof. In another example, the polyamidemay be residues of diaminobenzene, triaminobenzene, polyetherimine,piperazine or poly-piperazine or residues of a trimesoyl halide andresidues of a diaminobenzene. The selective barrier may also compriseresidues of trimesoyl chloride and m-phenylenediamine. Further, theselective barrier may be the reaction product of trimesoyl chloride andm-phenylenediamine.

In accordance with one or more embodiments, the selective barrier may becharacterized by a thickness adequate to impart desired salt rejectionand water permeability properties while generally minimizing overallmembrane thickness. In certain embodiments, the selective barrier mayhave an average thickness from about 50 nm and about 200 nm. Thethickness of the barrier layer is desired to be as limited as possible,but also thick enough to prevent defects in the coating surface. Thepractice of polyamide membrane formation for pressure drivensemi-permeable membranes may inform the selection of the appropriatebarrier membrane thickness. The selective barrier may be formed on thesurface of a porous support via polymerization, for example, viainterfacial polymerization.

Polymers that may be suitable for use as porous supports in accordancewith one or more embodiments include polysulfone, polyethersulfone,poly(ether sulfone ketone), poly(ether ethyl ketone), poly(phthalazinoneether sulfone ketone), polyacrylonitrile, polypropylene, poly(vinylfluoride), polyetherimide, cellulose acetate, cellulose diacetate, andcellulose triacetate polyacrylonitrile.

In accordance with one or more embodiments, the support layer may becharacterized by a thickness adequate to provide support and structuralstability to a membrane during manufacture and use while generallyminimizing overall membrane thickness. In certain embodiments, thepolymer support may have an average thickness from about 10 μm and toabout 75 μm. It is generally desirable for the support to be as thin aspossible without compromising the quality of the support surface forinterfacial polymerization of the barrier layer. The smoother thesupport layer is, the less thickness of support material is generallyrequired for this criterion. In at least some preferred embodiments,this layer is less than approximately 40 μm. In certain embodiments, theporous support comprises a first side (active side) with a firstplurality of pores, and a second side (support side) with a secondplurality of pores. In certain embodiments, the first plurality of poresand the second plurality of pores are fluidly connected to each other.In one embodiment, polymeric additives are dispersed within the poroussupport. Additives may enhance hydrophilicity, strength or otherdesirable properties.

In accordance with one or more embodiments, a thin-film compositemembrane may include a porous support comprising a first side with afirst plurality of pores, and a second side with a second plurality ofpores, wherein the average diameter of substantially all of the firstplurality of pores is between about 50 nm and about 500 nm, and theaverage diameter of substantially all of the second plurality of poresis between about 5 μm and about 50 μm. The purpose of the top layer isto allow for a high quality barrier to form by interfacialpolymerization or other deposition method, and to provide mechanicalsupport to a very thin barrier layer. The purpose of the remainder ofthe support structure is to be as open and as minimally tortuous aspossible, while being as thin as possible. Large pores towards thebottom may facilitate this purpose.

In accordance with one or more embodiments, a polymeric additive may bedispersed in the porous support. This addition may add strength, foulingresistance, hydrophilicity, or other desirable properties to the supportporous structure and materials. In the case of hydrophilic additions,very small quantities may be added. By way of example, between 0.1-1% ofPVP may be added to the polysulfone to enhance hydrophilicity in thestructure. A semipermeable selective barrier may be applied on the firstside of the porous support.

In certain embodiments, the membrane flux may be between about 15 andabout 25 gallons per square foot per day under operating conditions of1.5 M NaCl draw solution and a DI feed solution at 25° C. This high fluxis an indication of the effectiveness of the thin, open, porous, and lowtortuosity support layer in reducing resistance to diffusion of drawsolutes into the membrane support structure to provide driving force forflux in the form of osmotic pressure. This high flux is due in part alsoto the water permeability of the barrier layer.

In accordance with one or more embodiments, a method of making a forwardosmosis membrane may comprise providing a support structure including atleast a first layer and a second layer, applying a material to a firstlayer of the support structure to form a membrane support layer,applying a barrier material to the membrane support layer to form theforward osmosis membrane, and releasing the forward osmosis membrane byseparating the first layer of the support structure from the secondlayer of the support structure.

In accordance with one or more embodiments, a method of making a forwardosmosis membrane may comprise providing a support structure including atleast a thin support layer, adding a material to the support layer toform a membrane support structure, releasing the thin layer and materialstructure as one piece, and coating the membrane support structure witha barrier material to form the forward osmosis membrane.

In accordance with one or more embodiments, a method of facilitating aforward osmosis separation operation may comprise providing a supportstructure, applying a thin support layer to the support structure,applying a supporting material to the layer to form a porous membranesupport structure, applying a barrier layer to the porous membranesupport layer to form a forward osmosis membrane, and configuring theforward osmosis membrane in a forward osmosis membrane module such thatthe support structure may provide a turbulence spacer for a drawsolution or a feed solution supplied to the module during the forwardosmosis separation operation.

In accordance with one or more embodiments, a method of facilitating aforward osmosis separation operation may comprise providing a supportstructure, applying a supporting material to the structure to form aporous membrane support structure, applying a barrier layer to theporous membrane support layer to form a forward osmosis membrane, andconfiguring the forward osmosis membrane in a forward osmosis membranemodule such that the support structure may provide a turbulence spacerfor a draw solution or a feed solution supplied to the module during theforward osmosis separation operation.

In accordance with one or more embodiments, a forward osmosis membranemay comprise a fabric layer of less than about 75 microns, a supportlayer of less than about 50 microns applied on the fabric layer, and abarrier layer applied on the support layer. The forward osmosis membranemay have an overall thickness of less than about 125 microns. Thesupport layer may contain less than about 30 g/m² of material overall.In some embodiments, the supporting material may be applied in a coatingof between about 5 and 20 g/m². In some nonlimiting embodiments, thecoating may be between about 8 and 17.5 g/m². The combined weight of thesupport layer, support material, and barrier layer overall may bebetween 20 and 40 g/m². The support layer may be made in a wet laidprocess.

In some embodiments, the method may comprise rewetting the forwardosmosis membrane by immersion in water containing a solute whichenhances wetting. The solute may be a surfactant. In other embodiments,the method may further comprise rewetting the membrane by immersion inwater containing a low surface tension solvent. The solvent may comprisean alcohol. In some embodiments, the solvent may comprise isopropylalcohol, ethanol or methanol. In at least one embodiment, the supportstructure is embedded within the hydrophilic material. In someembodiments, the barrier material comprises polyamide.

In accordance with one or more embodiments, a forward osmosis membraneis disclosed. In some embodiments, the membrane may be a composite,generally including an active layer in conjunction with a support layer.Other parameters such as the mass percent of polymer used, choice ofsolvent and/or bath temperature may impact the degree to which thesupport layer is rendered hydrophilic, open or porous. The active layermay generally include any material capable of rejecting or otherwiseacting as a barrier to one or more target compounds present in a processstream brought into contact with the membrane. In at least oneembodiment, the active layer may comprise polyamide. Other materialscommonly used as membrane active layers may also be implemented.

A desired degree of cross-linking may be achieved within the activelayer, such as to improve the barrier properties of the membrane.Inducing cross linking in the polyamide layer is generally desirable toimprove salt rejection and overall performance. In accordance with oneor more embodiments, cross-linking is achieved in a manner such that thehydrophilic materials are not reduced in their performance, and aremaintained in a wet state throughout the manufacturing and treatmentprocess. In some embodiments, hot water annealing may be used tofacilitate cross-linking. In other embodiments, heat treatment may occurin one or more of the immersion steps of the membrane fabricationprocess, during or after the active layer deposition or formationprocess. In other embodiments, chemical treatment may be used. In atleast one embodiment, heat drying, such as oven drying, is not used. Insome such embodiments, the membranes will readily rewet by immersion inwater, and in some embodiments, they will rewet by exposure to a wettingagent in conjunction with water, such that they will be substantiallywet when ready for use. In some embodiments, the membranes may becharacterized as having a salt rejection of at least 99% or greater. Theforward osmosis membranes may generally be relatively thin andcharacterized by high porosity, low tortuosity and high wettability. Themembranes may find use in a variety of applications includingosmotic-driven water purification and filtration, desalination ofseawater, purification of contaminated aqueous waste streams, separationof various aqueous streams, osmotic power generation and the like.

In accordance with one or more embodiments, a polymer or other porousmembrane material may be deposited via various known techniques, such asphase inversion, on a thin woven or non woven or inorganic substrate togive a forward osmosis membrane with a very thin support structure. Insome embodiments, the substrate used for membrane fabrication mayinclude a multi-layered support. An ultrafiltration (UF) substrate maybe placed on a multi-layer woven or non-woven support, such that one ormore layers may be removable at the end of a membrane fabricationprocess prior to module construction. In at least some embodiments, thesize of the pores of the substrate material may be in the UF range, forexample, about 100 nm to about 1 um diameter, to facilitate properformation of an interfacial polymerization barrier layer on its surface.In some non-limiting embodiments, deposition may include phase inversionof polymers such as polysulfone or PAN. Layers which remain connected tothe UF substrate may be optimized for desirable characteristics such ashigh porosity, low tortuosity, thinness or other properties whichenhance diffusion to the UF layer. The materials may be but need not begenerally hydrophilic. In at least one embodiment, one or more layersmay serve as a draw solution or feed solution spacer. Inone-non-limiting embodiment, for example, a 100 nanometer layer ofpolyamide may be deposited on 0.5 mil layer of polysulfone on a 0.5 milpaper. In some embodiments, a barrier coated UF material may be placed,such as with phase inversion of a polymer, followed by an interfacialpolymerization of a barrier layer, on a woven or non-woven support suchthat after the manufacturing of the membrane, the support may be removedleaving only the UF and barrier material to be used in a module. In somenon-limiting embodiments, a UF layer may be deposited then coated with abarrier layer such as polyamide or polyamide urea. In at least oneembodiment, the substrate may include an intentionally separablesupport.

This support structure may be below the polymer, partially enclosed, orfully enclosed within it. In some embodiments, interfacialpolymerization or coating of a thin selecting layer which permits thepassage of water but not salts may then be implemented. In otherembodiments, deposition of a porous support layer on a removablematerial, such as delamination from a belt or other linear, mobilebacking material that is used to enhance material handling in thefabrication equipment, but which is not intended to become part of thefinished membrane product, may be used to create a very thin membranesupport structure, such that it has no backing. A thin barrier layer maythen be deposited on the surface of the paperless support. In someembodiments, the barrier layer may be deposited on the support prior tothe delamination of the support from the belt. In some embodiments apolymer may be coated on the surface of the support polymer, this toplayer acting as the salt rejection layer, either with or withoutsubsequent treatment. In other embodiments, the support layer itselffunctions as the barrier layer, having sufficient salt rejectionproperties for the application to which it will be applied.

In accordance with one or more embodiments, a belt or drum may be usedto facilitate casting upon thin supports. In at least one embodiment, abelt, such as a conveyor belt or like structure may be used as asubstrate replacement that remains primarily within the scope of themembrane manufacturing equipment. The belt may provide support for thedeposition of the support and barrier coating but may be retained andreused in the fabrication equipment rather than removed at the end anddiscarded or reused. In some embodiments, the belt or drum may bedesigned to provide a surface which retains a portion of a poroussupport material. The base of a porous support structure may bedisrupted when it is removed from the belt or drum due to retention ofpart of the porous support material. Such disruption may be beneficial.Retained porous support material may be removed prior to full rotationof the belt or drum to prevent accumulation of the material orinhibition of the effectiveness of the deposition and disruptionprocess.

In some embodiments, phase inversion of a polymer coated on or around amaterial intended primarily to give the polymer resistance todeformation with strain may be used to create a membrane support. Forexample, a very open and thin woven or non woven material may besurrounded by the polymer, rather than underneath it. Interfacialpolymerization of a rejecting polymer may then be carried out on thissupport structure.

In accordance with one or more embodiments, a forward osmosis membranemay be a hydrophilic phase inversion membrane on a woven or non wovenfabric. The hydrophilic material may be PAN in some non-limitingembodiments, alone or mixed with other monomers. The fabric layer may beof any desired thickness. In some non-limiting embodiments, the fabricmay be about 25 micrometers in thickness. The forward osmosis membranemay be further characterized by polyamide interfacial polymerization onits surface. A polyamide active layer may be applied so as to result ina membrane of any desired thickness. In some non-limiting embodiments,the membrane may be approximately 25 micrometers thick. The active layerof the forward osmosis membrane may be modified to enhance rejection ofdraw solutes. The support film may be nonwoven and made of any material,but thinness, high porosity, low tortuosity, and hydrophilicity aregenerally desirable. The thickness of the support film may vary. In someembodiments, the support film may be less than about 100 micrometers,less than about 80 micrometers, less than about 50 micrometers orthinner. In at least one embodiment, a porous polyester nonwoven supportfilm may be used as a substrate.

In accordance with one or more embodiments, a forward osmosis membranemay be formed by first creating a support layer. In some non-limitingembodiments, a thin fabric backing layer of less than about 30micrometers may be coated with a polysulfone solution of about 12.5% indimethylformamide. The effect of polysulfone thickness on the flux of aforward osmosis membrane is illustrated in accompanying FIG. 3. Lowerconcentrations of polysulfone may be used to further improve forwardosmosis membrane properties, including flux, as evidenced byaccompanying FIG. 1. In some embodiments, the amount of polysulfonecoating may generally be less than about 16 g/m² to minimize the impactof the support layer on diffusion. The application of a support layer ona typical reverse osmosis fabric backing of about 3.9 mils in thicknessmay result in much less than optimal forward osmosis flux as evidencedby accompanying FIG. 2.

The resulting support layer precursor may then be immersed in roomtemperature water causing the phase inversion of the polymer. Immersionin temperatures greater than 90F may be used to improve the pore sizecharacteristics of the support layer. This may produce a thin,microporous, open support structure with an embedded web giving thepolymer strength for rolling and handling. The active layer may then beapplied to the support structure. A non-limiting example of the coatingof this support structure with the active layer would be the immersionof the support in a solution containing polyamide or other desiredactive material. In one non-limiting embodiment, the support structuremay be immersed in a 3.4% solution of 1-3 phenylenediamine in roomtemperature water. The concentration of the solution may vary based ondesired characteristics of the applied active layer. Duration ofimmersion may also vary. In some non-limiting embodiments, the durationmay be less than about 5 minutes. In one specific embodiment, theduration of immersion may be about 2 minutes. Excess solution from thesurface of the membrane may be removed, for example with a roller or airknife.

The membrane may then be briefly immersed in another solution to inducethe polymerization of the polyamide rejecting layer by combination ofthe diamine in the aqueous phase and, for example, acid chloride in thenon-aqueous phase, at the surface of the support material where thephases meet. In some non-limiting embodiments, the membrane may beimmersed in the solution for about 2 minutes. In one non-limitingembodiment, a 0.15% solution of 98% 3,5 benzenetricarbonyltrichloride inIsopar® C or G at room temperature may be used. The membrane may thenremoved and the Isopar® allowed to evaporate from the membrane for aperiod of time, for example less than about 5 minutes. In someembodiments, the duration of the evaporation step may be about 2minutes. In some embodiments, immersion may take the form of a dipcoating process, such as one in which substantially only the surface ofthe membrane comes into contact with a solution. In other embodiments,the entire membrane may be submerged in the bath. In some embodiments, acombination of these techniques may be used, such as in a sequence ofdifferent immersion steps.

In accordance with one or more embodiments, a forward osmosis compositemembrane may be heat treated to condition the rejecting layer so as tocause cross-linking, which may be referred to as annealing. Drying outthe membrane, however, may in some cases cause detrimental shrinking ofthe void and pore structure. In accordance with one or more embodiments,conditioning may be achieved by a wet annealing step, for example,immersion in a hot water immersion bath. Any temperature water bath maybe used that is capable of resulting in a desired degree ofcross-linking and desired membrane performance, for example in terms ofsalt rejection. In some embodiments, as low of a temperature as possiblemay be desirable. In some non-limiting embodiments, a water bath belowabout 100° C. may be used. In one specific non-limiting embodiment, a95° C. water bath may be used. Duration in the water bath may also be inline with requirements to achieve the desired degree of cross-linking.In one specific embodiment, the hot water bath immersion occurs for 2minutes.

In other embodiments, the heat treatment of the membrane may occur inany or several immersion steps intended for other purposes, such asduring or after the active layer polymerization or deposition step.

In other embodiments, the heat treatment of the membrane may be done ina drying step, preceded by a solvent exchange involving a solvent with alower surface tension than water being exchanged with water byimmersion. In this way, the membrane properties may not be adverselyaffected and the membrane may retain the properties of being readilyrewet.

In other embodiments, the treatment of the rejecting layer may becarried out by a chemical treatment rather than a heating step, in whichcase the membrane would be kept wet throughout the treatment process.

In some embodiments, treating the membrane support layer with thepolyamide material to induce crosslinking in the polyamide layer to formthe forward osmosis membrane comprises chemical treatment. In otherembodiments, the treatment step involves hot water immersion, and thismay be referred to as wet annealing. In other embodiments, the heattreatment occurs at any point during or after the polyamide phaseinversion process, by maintaining a sufficient temperature in one ormore immersion steps. In other embodiments, the crosslinking is carriedout by dry annealing, but this is done after a solvent exchange step,soaking the wet membrane in a solution comprising another solvent with alower vapor pressure, so that “dry annealing” may be carried out withoutadversely affecting the membrane by such drying.

In accordance with one or more embodiments, a method of making a forwardosmosis membrane may comprise providing a thin woven or non-woven fabricbacking of less than about 50 micrometers in thickness. A support layerand a barrier layer may be applied to the fabric backing. The thin wovenor non-woven fabric support layer (top layer) may be characterized by aFrazier air permeability of greater than about 100 ft³/ft²/min. The thinwoven or non-woven fabric backing layer (bottom layer) may becharacterized by a Frazier air permeability of less than about 10ft³/ft²/min. In some non-limiting embodiments, a Frazier airpermeability of about 5 ft³/ft²/min for the bottom layer may bedesirable. In at least some embodiments, the thin woven or non-wovenfabric backing may be of less than about 30 g/m² basis weight. Thesupport layer applied to the fabric backing may be less than about 40micrometers in thickness. The barrier layer may be applied to thepolymeric support layer.

In some embodiments, a fabric backing layer and a support layer may beprovided using a polysulfone concentration of less than about 13%. Incertain non-limiting embodiments, the support structure may be modifiedwith a process quench temperature greater than about 95° F. The supportlayer of the forward osmosis membrane may also be modified using asolution with a hydrophilic agent, for example, polyvinylpyrrolidone.

In accordance with one or more non-limiting embodiments, a forwardosmosis membrane may comprise a fabric backing, a support layer of lessthan about 16 g/m² of polysulfone or polyacrilonitrile, and a barrierlayer.

In accordance with one or more embodiments, a forward osmosis membranemay be characterized by a flux of greater than about 20 gfd using 1.5MNaCl draw solution and deionized feed water at 25° C.

In accordance with one or more embodiments, a forward osmosis membranemay be characterized by a salt rejection of greater than about 99% using1.5M NaCl draw solution and deionized feed water.

In accordance with one or more embodiments, a polysulfone solution ofless that about 13% may be applied to enhance one or more properties ofa forward osmosis membrane. For example, in some non-limitingembodiments, less than about 16 g/m² of polysulfone may be applied on afabric backing layer of a forward osmosis membrane.

In accordance with one or more embodiments, a method of making a forwardosmosis membrane may comprise providing a support structure, applying ahydrophilic material to the support structure to form a membrane supportlayer, applying a polyamide material to the membrane support layer,immersing the membrane support layer with the applied polyamide materialin water, carrying out a solvent exchange by immersion of the membrane,and dry annealing the membrane support layer with the polyamide materialto form the forward osmosis membrane.

In accordance with one or more embodiments, a forward osmosis membranemay be processed on production lines currently used in the manufactureof reverse osmosis, membranes.

In accordance with one or more embodiments the method by which a thinforward osmosis membrane is processed may include the use of anintegrated drive system to control tension in discrete sections of amachine. Supplemental manual or automated web steering devices may alsobe implemented. In some embodiments, machine design may allow for nomore than about a 10% tension drop per zone to reduce over tensioning ofthe membrane which may lead to creasing and folding over of themembrane. Membrane tensions of less than about 1 pound per linear inchmay prevent creasing of the membrane. The design of the machine may alsogenerally limit the free span of unsupported membrane to less than onehalf the web width in areas in which the membrane is submersed. In somenon-limiting embodiments, machine design may also include opticalalignment at a tolerance of about 0.001 inches per linear foot of rollerwidth to prevent creasing and folding over of the membrane.

In accordance with one or more embodiments, various techniques disclosedherein may be used to make membranes for forward osmosis applications.In accordance with one or more embodiments, various techniques disclosedherein may also be used to make membranes for applications involvingpressure retarded osmosis. In some embodiments, pressure retardedosmosis may generally relate to deriving osmotic power or salinitygradient energy from a salt concentration difference between twosolutions, such as a concentrated draw solution and a dilute workingfluid. Within pressure retarded osmosis, a draw solution may beintroduced into a pressure chamber on a first side of a membrane. Insome embodiments, at least a portion of the draw solution may bepressurized based on an osmotic pressure difference between the drawsolution and a dilute working fluid. The dilute working fluid may beintroduced on a second side of the membrane. The dilute working fluidmay generally move across the membrane via osmosis, thus increasing thevolume on the pressurized draw solution side of the membrane. As thepressure is compensated, a turbine may be spun to generate electricity.A resulting dilute draw solution may then be processed, such asseparated, for reuse. In some embodiments, a lower-temperature heatsource, such as industrial waste heat may be used in or facilitate apressure retarded osmosis system or process.

The function and advantages of these and other embodiments will be morefully understood from the following example. The example is intended tobe illustrative in nature and is not to be considered as limiting thescope of the systems and methods discussed herein.

EXAMPLE

The disclosed approaches have been demonstrated on an existing 40 inchmembrane production machine with 1.7 mil top PET alone or with 2.3 milbacking material which is later removed. Techniques using a single layersupport are generally referred to herein as Gen 1, while techniquesusing a bilayer approach are generally referred to herein as Gen 2.

FIG. 1 presents flux data as a function of polysulfone concentration.Flux was measured at various polysulfone concentrations with otherparameters remaining constant. In accordance with a substantiallyinverse relationship, flux increased as polysulfone concentrationdecreased. Experimental data has shown improved forward osmosis membraneflux with polysulfone concentrations in the range of about 9% to about13%. Membranes used were Gen 1, but with a thicker 4 mil PET.

FIG. 2 presents data indicating an inverse relationship between membranethickness and flux. Flux decreased with increasing membrane thickness,generally indicating the desirability of thin membranes. Otherparameters were kept constant. Flux was also measured for variouspolysulfone loadings with other parameters remaining constant. Membranesused were Gen 1.

FIG. 3, consistent with FIG. 1, indicates that flux decreased withincreasing polysulfone loading. Membranes used were Gen 1.

FIG. 4 depicts flux data collected using two membranes, depicted as Gen2A and Gen 2B, both manufactured in accordance with the decreased PS(polysulfone) concentration, decreased PS loading, increased porosity,and bilayer approach disclosed herein, in comparison to flux datacollected from a membrane made with a single supporting layer depictedas Gen 1, where only the benefits of decreased PS concentration,decreased PS loading, and increased porosity were employed, but nobilayer manufacturing technique. The test method involved using 1.5MNaCl draw solution on the supporting side of the membrane and eitherdeionized water or 0.5 M NaCl solution on the feed side. Flux wasmeasured as the mass change in the draw solution over time. Asindicated, the disclosed bilayer technique was associated with superiorflux results attributable to the reduced thickness and other parametersand characteristics associated with the bilayer techniques disclosedherein. In particular, all of these parameters worked together to yielda superior result. Not shown is an experimental run involving a membraneproduced via the bilayer technique without decreased PS concentration,decreased PS loading, and increased porosity, which producedsignificantly less flux than the Gen 2A and 2B samples shown here. Alsonot shown is the very low flux that was produced by using a membranemade by techniques used in pressure driven semi-permeable membranemanufacturing. Conventional RO membranes in this experiment producedless than 1 GFD of flux.

FIG. 5 depicts the flux results of a membrane made in accordance withthe bilayer technique described herein in comparison to a membrane madewith a single supporting layer. The membrane which produced the highestflux was manufactured by first combining two layers of PET. The toplayer of PET was 1.5 mils in thickness with a basis weight of 15 gramsper square meter and was combined with a backing PET with a thickness of2.3 mils and a basis weight of 49 grams per square meter. The topthinner layer of PET was coated with a 12.5% solution of polysulfone anddimetylformamide resulting is a polysulfone coating weight of 21 gramsper square meter. The described support was then processed usingmembrane chemistry of an acid chloride and amine to produce the forwardosmosis polyamide membrane. Following the membrane formation, the bottomPET layer was removed prior to module construction. The resultantmembrane and support had a basis weight of approximately 35 grams persquare meter and a total thickness of 95 microns.

The test method was to use 6 M ammonium salt draw solution on thesupporting side of the membrane and varying concentrations of NaClsolution on the feed side at a temperature of 50° C. Flux was measuredas the mass change in the draw solution over time. As indicated, thedisclosed bilayer technique was associated with superior flux resultsdue to the reduced thickness and other parameters and characteristicsassociated with the bilayer technique disclosed herein. Particularly,this illustrates the capability of treating very high salinity feedsusing membranes in a way that has not in the past been possible due tothe benefits of the membrane manufacturing techniques described herein.The data points presented for 0.5M feed are experimental, and theremaining data represents an extrapolation using accepted modelingtechniques. Not shown is the very low flux that is produced by using amembrane made by techniques used in pressure driven semi-permeablemembrane manufacturing. Conventional RO membranes would producenegligible flux in the high salinity feed scenarios.

FIG. 6 presents an SEM image of a membrane manufactured in accordancewith one or more bilayer techniques described herein. The pore structureis substantially open at a base region of the polymer support layerwhich previously interfaced with the backing layer prior to removalthereof. The polyamide barrier layer, the porous support layer, thetopmost PET and backing PET are all shown.

Having now described some illustrative embodiments, it should beapparent to those skilled in the art that the foregoing is merelyillustrative and not limiting, having been presented by way of exampleonly. Numerous modifications and other embodiments are within the scopeof one of ordinary skill in the art and are contemplated as fallingwithin the scope of the invention. In particular, although many of theexamples presented herein involve specific combinations of method actsor system elements, it should be understood that those acts and thoseelements may be combined in other ways to accomplish the sameobjectives.

It is to be appreciated that embodiments of the devices, systems andmethods discussed herein are not limited in application to the detailsof construction and the arrangement of components set forth in thefollowing description or illustrated in the accompanying drawings. Thedevices, systems and methods are capable of implementation in otherembodiments and of being practiced or of being carried out in variousways. Examples of specific implementations are provided herein forillustrative purposes only and are not intended to be limiting. Inparticular, acts, elements and features discussed in connection with anyone or more embodiments are not intended to be excluded from a similarrole in any other embodiments.

Those skilled in the art should appreciate that the parameters andconfigurations described herein are exemplary and that actual parametersand/or configurations will depend on the specific application in whichthe systems and techniques of the invention are used. Those skilled inthe art should also recognize or be able to ascertain, using no morethan routine experimentation, equivalents to the specific embodiments ofthe invention. It is therefore to be understood that the embodimentsdescribed herein are presented by way of example only and that, withinthe scope of the appended claims and equivalents thereto; the inventionmay be practiced otherwise than as specifically described.

Moreover, it should also be appreciated that the invention is directedto each feature, system, subsystem, or technique described herein andany combination of two or more features, systems, subsystems, ortechniques described herein and any combination of two or more features,systems, subsystems, and/or methods, if such features, systems,subsystems, and techniques are not mutually inconsistent, is consideredto be within the scope of the invention as embodied in the claims.Further, acts, elements, and features discussed only in connection withone embodiment are not intended to be excluded from a similar role inother embodiments.

The phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. As used herein, theterm “plurality” refers to two or more items or components. The terms“comprising,” “including,” “carrying,” “having,” “containing,” and“involving,” whether in the written description or the claims and thelike, are open-ended terms, i.e., to mean “including but not limitedto.” Thus, the use of such terms is meant to encompass the items listedthereafter, and equivalents thereof, as well as additional items. Onlythe transitional phrases “consisting of” and “consisting essentiallyof,” are closed or semi-closed transitional phrases, respectively, withrespect to the claims. Use of ordinal terms such as “first,” “second,”“third,” and the like in the claims to modify a claim element does notby itself connote any priority, precedence, or order of one claimelement over another or the temporal order in which acts of a method areperformed, but are used merely as labels to distinguish one claimelement having a certain name from another element having a same name(but for use of the ordinal term) to distinguish the claim elements.

1. A method of making a forward osmosis membrane, comprising: providinga support structure comprising a bilayer substrate including first andsecond separable layers; applying a material to the first layer of thebilayer substrate of the support structure to form a membrane supportlayer; applying a barrier material to the membrane support layer to formthe forward osmosis membrane; and separating the second layer of thebilayer substrate from the first layer of the bilayer substrate to formthe forward osmosis membrane.
 2. The method of claim 1, wherein thefirst layer of the bilayer substrate has a Frazier air permeability ofgreater than about 50 cfm/ft² min and a thickness of less than about 2mils.
 3. The method of claim 1, wherein the material applied to thefirst layer of the bilayer substrate comprises a polymer loading ofbetween about 5 and 20 g/m².
 4. The method of claim 1, wherein thesupport layer has an overall thickness of less than about 50 microns. 5.The method of claim 1, wherein the barrier material comprises asemipermeable material.
 6. The method of claim 5, wherein the barriermaterial comprises a polymer.
 7. The method of claim 6, wherein thebarrier material comprises a polyamide urea, polypiperazine, or a blockco-polymer.
 8. The method of claim 1, wherein the bilayer substratecomprises a polymeric paper.
 9. The method of claim 8, wherein thebilayer substrate comprises polyethylene terephthalate or polypropylene.10. The method of claim 1, further comprising rewetting the forwardosmosis membrane.
 11. The method of claim 1, further comprising addingan additive to one or more layers of the membrane.
 12. The method ofclaim 1, wherein the step of separating the second layer of the bilayersubstrate from the first layer of the bilayer substrate comprisesmodifying a pore structure in the support layer.
 13. The method of claim1, further comprising reusing the second layer of the bilayer substrate.14. A method of making a forward osmosis membrane, comprising: providinga support structure comprising a bilayer substrate including first andsecond separable layers; applying a material to the first layer of thebilayer substrate to form a membrane support layer, such that thematerial penetrates at least a portion of the first layer of the bilayersubstrate and only partially penetrates the second layer of the bilayersubstrate; applying a barrier material to the membrane support layer toform the forward osmosis membrane; separating the second layer of thebilayer substrate from the first layer of the bilayer substrate; andmodifying a pore structure of the support layer.
 15. A method of makinga forward osmosis membrane, comprising: providing a support structurecomprising a bilayer substrate including first and second separablelayers, wherein the first layer has a Frazier air permeability ofgreater than 100 cfm/ft²-min; applying a material to the first layer ofthe bilayer substrate to form a membrane support layer; applying abarrier material to the membrane support layer to form the forwardosmosis membrane; and separating the second layer of the bilayersubstrate from the first layer of the bilayer substrate to form theforward osmosis membrane.